In general, lithographic printing involves a step of applying water to a hydrophilic non-image areas of a printing plate to prevent adherence of oily printing ink and a step of feeding oily printing ink to oleophilic image areas of the printing plate. However, maintaining of the delicate balance between the amount of water applied to the plate and the amount of ink fed to the plate is difficult and needs a skilled worker.
In order to overcome these problems of conventional lithography, waterless lithographic printing plate capable of printing in the absence of dampening water have been provided. Waterless lithographic printing plates have oil repellant areas and oleophilic areas. Oily ink is applied to the plate and adheres only to the oleophilic areas and an ink image thus formed on the plate is transferred to paper. One method practically used comprises imagewise exposing to light a light-sensitive material having a silicone rubber layer and a light-sensitive layer composed of a photosensitive resin to make difference in adhesion between the silicon rubber layer and the light-sensitive layer in the exposed area from the non-exposed areas and removing the imaging areas by a wet development processing to prepare a lithographic printing plate. This method requires contact imagewise exposure using a light source having a short wavelength and a high power due to low-sensitivity of the light-sensitive element and the wet development processing. Therefore, this method has problems in simplicity, rapidness and laborsaving and is very difficult to apply to the preparation of lithographic printing plate accepting a recent image-forming system using a digital signal, i.e. a digital direct printing plate.
A system has been commercialized by Heiderberg Co., Ltd. wherein a material comprising a heat-sensitive layer containing a substance capable of converting radiation into heat and a silicon layer provided thereon is subjected to scanning exposure by a laser beam corresponding to a digital signal to destroy the silicon layer together with the heat-sensitive layer using the heat generated in the exposed portion, followed by removing these layers in the exposed portion by a dry development processing thereby providing a waterless printing plate.
According to the system, writing by a laser beam using a heat mode and a dry development processing are employed. However, a laser writing device of high power is necessary because of low sensitivity of the recording material which leads to increase in a size of apparatus, a period of plate-making and a cost of the system.
JP-A-47-19305 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-49-19904, JP-A-59-125752 and JP-A-62-160466 each discloses a method capable of image-forming simply in an apparatus of a small size using an electrophotographic light-sensitive element suitable for scanning exposure by a semiconductor laser beam of a low power. On the electrophotographic light-sensitive element is provided a silicon layer and then an oleophilic toner image is formed thereon by an electrophotographic process to prepare a waterless printing plate.
However, adhesion of the toner image portion to the silicon layer is poor in the printing plate and the image portion is apt to be damaged by tack of ink supplied which results in the occurrence of image failure. Thus printing durability of the plate is very low.
In order to improve the printing durability there have been proposed methods for increasing adhesion between the toner image portion and the silicon layer. For example, there are a method wherein a unhardened silicon rubber layer is provided and after the formation of toner image, the silicon rubber is hardened as described, for example, in JP-A-50-53110 and JP-A-52-105003, and a method using a reactive group containing silicon rubber layer as described, for example, in JP-A-52-29305, JP-A-56-83750 and JP-A-57-178893. However, these methods are still insufficient in the adhesion for the practical purpose.
JP-A-49-121602 discloses a method comprising forming an image composed of dry toner on a support for lithographic printing plate by a PPC copying machine such as a laser printer using a semiconductor laser of a low power or a printer of heat-sensitive transfer, providing a silicon layer on the whole surface of the support, hardening the silicon layer and then selectively removing the silicon layer on the image portion upon a wet development processing using a solvent to prepare a printing plate.
Also, JP-A-3-118154 discloses a method comprising forming a light absorber-containing image or a non-adhesive image using dry toner on a support for lithographic printing plate by a PPC copying machine such as a laser printer using a semiconductor laser of a low power or a printer of heat-sensitive transfer, providing a silicon layer on the whole surface of the support, hardening the silicon layer and then selectively removing the silicon layer on the image portion upon a dry development processing using heat or mechanical means to prepare a printing plate.
According to these methods described in JP-A-49121602 and JP-A-3-118154, poor adhesion of toner image to a silicon layer occurred in the printing plate prepared by forming the toner image on the silicon layer as described hereinbefore can be solved. Further, a simple dry process can be used for removing the silicon layer on the image portion in the method described in JP-A-118154.
However, these methods still have problems. Specifically, since adhesion between the silicon layer and the support in the non-image portion is insufficient and releasability of the silicon layer depends on a conversion rate of radiation to heat of a dye or pigment employed, a difference of adhesion between the silicon layer and the support in the non-image portion from adhesion between the image portion and the silicon layer is small in fact. Accordingly, it is difficult to selectively remove the silicon layer on the image portion in fine image regions, particularly by a dry process, and thus these methods are not sufficient for providing constantly good printing plates.
Further, there is a limit to forming a highly accurate image using a PPC copying machine or printer of heat-sensitive transfer as well known in the art and a printing plate having excellent image qualities is hardly obtained.
Recently, a printing system providing prints of highly accurate full color image in a simple, rapid and laborsaving manner including edition in a workstation and digital data processing has been highly desired. However, such a desire cannot be answered by the techniques described above.